It is well known that control of “skin” formation in compacted graphite iron castings and ductile iron castings is critical to their mechanical performance, particularly fatigue strength. The formation of skin with degenerated graphite morphology in the casting periphery can decrease the strength and ductility of the casting to a far greater extent than the relative thickness of the skin might suggest because graphite flakes in particular can act as stress concentrators and crack initiation sites. The ability to control the degradation of vermicular or spheroidal graphite in the casting skin requires a fundamental understanding of the effects of metal composition and treatment, inoculant fade in the furnace prior to pouring and in the mold after pouring, mold/metal reactions inside the mold and cooling rates during solidification of the metal.
Generation of test castings is a common methodology used by industry and academic research groups in an effort to correlate data from laboratory or small scale metal casting operations to production operations and specific commercial castings. Many examples exist for test casting designs and geometries to address specific topics, for instance mold erosion, metal penetration, veining, lustrous carbon formation, etc., but none that specifically address the effect of skin formation in compacted graphite iron and/or ductile iron as a result of molding materials, mold/metal interface reactions and cooling rates of the metal upon pouring and filling the mold.
Although the phenomenon of skin formation and its causes are acknowledged and the effect is commonly probed via metallurgical microstructure evaluation, a suitable test which not only shows the effect but also allows one to investigate contributors such as cooling rates and the effect of molding materials, including aggregate, sand additives, binders, refractory coatings and the like does not exist.